Worldwide, over 80% of HIV infections occur as a result of vaginal transmission. There is a desperate need for a safe and effective microbicide than can prevent the transmission of HIV to women. The goal of Research Project 3 is to identify the optimal combination and formulation of fusion inhibitors that demonstrates the greatest level of safety, efficacy, and duration of protection against vaginal HIV transmission using the nonhuman primate model of vaginal transmission. Over the last several years, we have demonstrated that several fusion inhibitors can completely protect against vaginal transmission of multiple simian/immunodeficiency virus (SHIV) strains that are highly relevant to HIV-1 transmission. We now propose to determine optimal combinations of these inhibitors that provide the greatest level of efficacy against multiple strains and repeated challenge with divergent strains of virus. We will also test sustained delivery systems (vaginal rings) designed to confer extended protection in mucosal tissues after a single application. Finally, we will rigorously assess the safety of these inhibitors and delivery systems to determine how long these may be applied without inducing inflammation or adverse effects. Our experiments are thus designed to test the efficacy and safety of fusion inhibitors, alone, in combination, and particularly in sustained release formulations to identify the optimal combination of fusion inhibitors that can be safely and economically used as a microbicide to slow the spread of HIV infection, particularly in areas where the epidemic is spreading the fastest. The specific aims of Research Project 3 are to: Specific Aim 1. Determine the most effective combination of fusion inhibitors that provides optimal protection against mucosal transmission of diverse strains of SIV/SHIV in macaques; Specific Aim 2. Compare the efficacy and duration of protection of fusion inhibitors administered in sustained delivery devices in vivo, and; Specific Aim 3. Assess the safety of multiple dosing and sustained release formulations of fusion inhibitors in both the vaginal and rectal mucosa. The ultimate goal of this project is to identify a combination of HIV fusion inhibitors that can be safely applied in a sustained release formulation that will consistently protect women against repeated exposure to highly divergent strains of HIV.